Human motor unit activity during the onset of muscle fatigue in submaximal isometric isotonic contraction

Abstract

The onset of fatigue has been studied by relating motor unit activity and surface myoelectric activity during constant force isometric contraction in man. The surface (global) EMG of the biceps brachii was recorded using bipolar electrodes. The global EMGs of the other elbow flexors were also recorded. Motor unit activity was recorded simultaneously at three points of the biceps brachii by using wire electrodes. The time course of the integrated surface EMG showed that fatigue occurred right at the beginning of the contraction. The increase of the integrated EMG was especially important when the value of the force maintenance was high. The time course of this increase was close to that of the output of the spikes recorded by the wire electrodes. No matter what the value of the force maintenance, MU recruitment occurred throughout the contraction. The existence of an increase in the MU firing frequently depended on the value of this force. Contractile element fatigue is believed to provide the explanation of these results, while neuromuscular junction fatigue may perhaps be more important near the time limit.

Effects of taurine supplementation following eccentric exercise in young adults

“…The maximal isometric strength (MIF) was measured twice (1 min between the measurements) at an elbow flexion joint of 90°for 4 s. The peak force of each 4 s value was determined, and the mean of the two measurements was used for analysis (MaTon 1981).…”

Dissociation of the electrical and contractile properties in single human motor units during fatigue

“…The progressive enlargement in its rectified integrated area during submaximal fatiguing contraction is thought to primarily reflect an increase in the number of recruited motor units (MUs) and in their firing rates. 7,10,14 A third possibility is that the size of the motor unit action potential (MUAP) might change during fatiguing contraction. Such possibility indeed has been shown in animal studies, 1,8,18 but has not been systematically investigated in humans.…”

Assessment of Muscular Fatigue with Electromyography on Lower Back and Leg Muscles during Continuous Uphill and Downhill Load Carriage Task

“…In both these applications muscles contracted dynamically and briefly. There are other studies where decrease in frequency was observed which resulted in fatigue could be explained as central or peripheral in nature [27][28][29] . These observations corroborated with the decrease in the frequency during UH load carriage by Indian soldiers of the present study.…”